The raw natural gas, as extracted from wells, often contains acid compounds, in particular carbon dioxide and hydrogen sulfide. The natural gas may also contain moisture. The acid compounds, in combination with the moisture make the gas aggressive to the materials of the equipment in which the gas has to be stored and/or transformed. Furthermore, the acid compounds make the gas unsuitable for a large number of uses.
Nevertheless, the moisture may cause ice and hydrocarbon hydrates to form, when the gas is expanded after extraction. The ice can block passageways, and damage the equipment and the piping.
The water that is present in the gas may also form corrosive pools within the gas pipelines.
The same problems also occur with other fuel-gas mixtures, for example light gas fractions from crude oil atmospheric distillation.
Therefore, it is often necessary to remove acid compounds and/or moisture from hydrocarbon fuel gas, in particular from natural gas, immediately after extraction.
For removing the acid compounds, i.e. for “sweetening” the gas, absorption operations are normally carried out in which the gas is absorbed into an alkali-containing liquid, for example into a solution of an organic base that is able to chemically combine with the adsorbed compounds.
For dehydrating the natural gas that is extracted from a well at a pressure normally set between 1 and 300 bar absolute, processes are normally used of absorption into a hygroscopic liquid, preferably into a regenerable liquid, for instance into a glycol such as triethylene glycol (TEG).
According to the most common technique, the gas is treated first in a sweetening tower, and then in a dehydration tower.
A typical sweetening and dehydration combined process is described, for instance, in U.S. Pat. No. 4,150,962. This process provides an amine-based liquid in which the acid compounds are absorbed, and a glycol for the dehydration. As also disclosed in WO 2011/121423, the sweetening and dehydration steps are carried out in respective absorption chambers that are defined within a tower, and that are separated by an inner longitudinal partition wall.
In many cases, the flowrate of the gas that has to be treated changes remarkably with time. In particular, in the case of natural gas wells, the extraction flowrate significantly decreases during the life of the well. The sweetening and/or dehydrating apparatus of the type described in the cited documents comprises absorption towers or chambers in which the passage cross-section is selected on the basis of the maximum gas flowrate that the well can produce. Such towers cannot effectively treat a very low flowrate, such as the flowrate that may generally be extracted after even a few years of life of the well.
This is a particularly relevant problem if packed towers are used. Packed towers, in fact, are less sensitive to process gas flowrate variation than plate towers are. Packed towers are a substantially an unavoidable choice in the case of towers to be installed on deepwater platforms, such as Tension Leg Platform (TLP), or on other floating facilities, i.e. Floating Platforms or generically Floating Production Systems (FPO). In fact, the movement of the platforms caused by the waves does not allow to operate plate towers, or makes it disadvantageous, because plate towers do not allow a steady liquid hold-up to be maintained on the trays.
A relevant flowrate change may occur also when treating hydrocarbon gas such as refinery or cookery gas fractions, petrochemical processes gas streams, chemical synthesis gas or biogas that are produced by degradation and/or fermentation processes. Particularly low gas flowrates may occur, for instance, in case of plants that work at a reduced rate.
GB 2111852 describes an apparatus to cause a contact between a gas and a liquid, which comprises: an outer shell with a spherical shape or a closed end cylindrical shape; at least one partition positioned vertically within the shell, in order to form at least two separate spaces within the shell; a gas-liquid contact means mounted in each space; openings for introducing and withdrawing the gas and/or a liquid into/from such spaces. The apparatus also comprises gas and/or liquid passages that connect such spaces in a series, parallel or in series/parallel arrangement. The apparatus is conceived to limit the height-to-diameter ratio, in particular, in order to resist seismic forces and also to facilitate transport. In case of a cylindrical shell, the height-to-diameter ratio is lower than 7, preferably it is lower than 5. This is clearly due to particular earthquake conditions that are likely to take place in Japan, whereas conditions are known in which said ratio could be exceeded. This is obtained by placing side-by-side tower portions that, in a single-tower arrangement, would be arranged as the prolongation of each other. Accordingly, this way to reduce the height of the tower unavoidably increases its overall lateral dimensions, which is a disadvantage in an installation on structures that have a limited space availability.
Such an apparatus is not well-suited for sweetening/dehydration operations of a large number of gases that require such treatment. For example, most active extraction units, both gas and gas/oil extraction units, produce a gas streams that contain high amounts of undesirable substances and, in particular, hydrogen sulfide: this is the case of the gas from Caspian Sea extraction units (Kashagan), which may contain H2S amounts up to 18-20% in moles. In these cases, more complex sweetening towers are required, which have a large number of theoretical plates, in a non-extreme case of gas produced by the above-indicated extraction units up to 24 plates may be required, yet in a relatively mild treatment to obtain a gas that is not intended for specific catalytic chemical processes, i.e. a treatment in which a purity degree of about 5-6 ppm H2S may be enough, as in the case of a gas to be burnt in an industrial or home combustion plant.
In this case, if a plate height of about 500 mm is assumed, the mass exchange zone alone be as high as about 12-15 meters, which can bring at an overall tower height of about 15-18 meters, considering top and bottom distribution devices, and the shell portions and bottoms that are needed for them. A maximum height to diameter ratio of 7:1, as in GB 2111852, would mean a minimum diameter of 2.1-2.6 meters; such a value may be largely sufficient in comparison to what is required by a flowrate that may be reasonably treated in a sweetening unit. Even larger heights and therefore the pressure and flowrate being the same, even larger height-to-diameter ratios could be required in case of a treatment to obtain a hydrocarbon gas that are suitable for particular subsequent process conditions, as in the case of the gas to be used for olefins production, in which H2S concentration higher than 1 ppm cannot normally be tolerated.
Furthermore, GB 2111852 does not specifically relate to sweetening/dehydration processes of hydrocarbon gases. It relates to processes where a single operation is carried out, in which a treatment liquid, for example a sweetening liquid, is fed according to a parallel arrangement. In an embodiment, a sweetening/regeneration process is carried out in side-by-side chambers, which may lead to important problems due to unwanted heat exchange and to unfavourable temperature profiles, since the sweetening temperature of processes is normally quite lower than the temperature at which the corresponding absorption liquid is regenerated, as in the case of amines.
Furthermore, the gas and liquid passageway according to the above-mentioned document, as shown in the drawings, should be made with ducts arranged within the shell, which can disturb the gas and liquid flow and cause a loss of efficiency of the apparatus.
FR 2 776 206 describes an air distillation apparatus in which distillation towers comprise coaxial or sector-shaped portions, and are connected to each other. However, the operating pressure and temperature differences at which the portions operate are limited.
U.S. Pat. No. 5,800,788 describes an SO3 into H2SO4 absorption tower that is internally parted into a plurality of sections, in particular into three sections. Even in this case, the different sections work at substantially the same operating conditions, in particular the operating pressures are close to atmospheric pressure, while the operating temperature are substantially set between 60 and 80° C.
WO 98/32523 describes a gas scrubber that is internally parted into a plurality of chambers, in particular into two chambers. However, all the chambers operate at a substantially atmospheric pressure. The possible temperature differences between the chambers do not give origin to any particular force acting on the inner partition wall, since the latter can freely elongate towards its own upper end.
U.S. Pat. No. 4,198,387 describes a method and an apparatus for selectively removing H2S from a CO2-containing gas whose flowrate may be adjusted. The use is provided of a plurality of conventional absorption towers, in particular of two towers, which operate in parallel, where each tower has respective serially connected absorption volumes. This device cannot solve the problem of reducing the surface extension, and requires high installation costs.